1. Field of the Invention
The invention relates to a power receiving device, a power transmitting device, a vehicle and a contactless power supply system and, more particularly, to a contactless power supply technique for transferring electric power using electromagnetic resonance.
2. Description of Related Art
Vehicles, such as electric vehicles and hybrid vehicles, become a focus of attention as environmentally friendly vehicles. These vehicles each include an electric motor that generates running driving force and a rechargeable electrical storage device that stores electric power supplied to the electric motor. Note that the hybrid vehicles include a vehicle that further includes an internal combustion engine together with an electric motor as a power source, a vehicle that further includes a fuel cell together with an electrical storage device as a direct-current power supply for driving the vehicle, and the like.
In recent years, wireless power transmission that does not use a power cord or a power transmission cable becomes a focus of attention as a method of transmitting electric power from a power supply outside a vehicle to such a vehicle. Three leading techniques are known as the wireless power transmission technique. The three leading techniques are power transmission using electromagnetic induction, power transmission using electromagnetic wave such as a microwave and power transmission using a resonance method.
The resonance method is a contactless power transmission technique such that a pair of resonators (for example, a pair of resonance coils) are resonated in an electromagnetic field (near field) to thereby transmit electric power via the electromagnetic field. The resonance method is able to transmit large electric power of several kilowatts over a relatively long distance (for example, several meters).
Japanese Patent Application Publication No. 2009-130940 (JP 2009-130940 A) describes a configuration that, in a vehicle of which a mounted electrical storage device is chargeable from a power supply outside the vehicle via a charging cable, the residual electric charge of a capacitor provided between a pair of power lines that receive charging electric power is discharged by a discharging resistor for discharging the residual electric charge of a capacitor connected to an in-vehicle inverter.
At the time of transferring electric power in the power supply system, when there is an abnormality that a power receiving device cannot receive electric power, power transmission from a power transmitting device needs to be quickly stopped. However, in power transfer using a resonance method, electromagnetic energy resulting from resonance remains at the portions of resonance coils that are carrying out electromagnetic resonance. Therefore, even when the power receiving device stops receiving electric power, the remaining energy can be transmitted to the power receiving device.
Then, electric power is further supplied to the device having an abnormality or electric power that has nowhere to go continues to resonate between the power transmitting device and the power receiving device. This may cause degradation or breakage of devices or may influence surroundings.